It was just the type of event that many in the nanotechnology community have feared – and warned against. In late March, six people went to the hospital with serious (but nonfatal) respiratory problems after using a German household cleaning product called Magic Nano. Though it was unclear at the time what had caused the illnesses – and even whether the aerosol cleaner contained any nanoparticles – the events reignited the debate over the safety of consumer products that use nanotechnology.

The number of products fitting that description has now topped 200, according to a survey published in March by the Project on Emerging Nanotechnologies in Washington, DC. Among them are additives that catalyze combustion in diesel fuel, polymers used in vehicles, high-strength materials for tennis rackets and golf clubs, treated stain-resistant fabrics, and cosmetics. These products incorporate everything from buckyballs – soccer ball-shaped carbon molecules named after Buckminster Fuller – to less exotic materials such as nanoparticles of zinc oxide. But they all have one thing in common: their “nano” components have not undergone thorough safety tests.

[Click here for a table of recent findings on toxicity associated with nano materials and devices.]

Nanoparticles, which are less than 100 nanometers in size, have long been familiar as by-products of combustion or constituents of air pollution; but increasingly, researchers are designing and synthesizing ultrasmall particles to take advantage of their novel properties. Most toxicologists agree that nanoparticles are neither uniformly dangerous nor uniformly safe, but that the chemical and physical properties that make them potentially valuable may also make their toxicities differ from those of the same materials in bulk form.

One of the reasons for concern about nanoparticles’ toxicity has to do with simple physics. For instance, as a particle shrinks, the ratio of its surface area to its mass rises. A material that’s seemingly inert in bulk thus has a larger surface area as a collection of nanoparticles, which can lead to greater reactivity. For certain applications, this is an advantage; but it can also mean greater toxicity. “The normal measure of toxicity is the mass of the toxin, but with nanomaterials, you need a whole different set of metrics,” says Vicki Colvin, a professor of chemistry at Rice University in Houston and a leading expert on nanomaterials.

Beyond the question of increased reactivity, the sheer tininess of nanoparticles is itself a cause for concern. Toxicologists have known for years that relatively small particles could create health problems when inhaled. Researchers have found evidence that the smaller particles are, the more easily they can get past the mucus membranes in the nose and bronchial tubes to lodge in the alveoli, the tiny sacs in the lungs where carbon dioxide in the blood is exchanged for oxygen. In the alveoli, the particles face the white-cell scavengers known as macrophages, which engulf them and clear them from the body. But at high doses, the particles overload the clearance mechanisms.

It is the potential growth, however, of technologies involving precisely engineered nanoparticles, such as buckyballs and their near cousins, carbon nanotubes, and the use of these new materials in consumer products that has made the question of toxicity particularly urgent.

In addition to questions about how easily nanoparticles can penetrate the body, there is also debate over where they could end up once inside. Günter Oberdörster, a toxicologist at the University of Rochester, found that various kinds of carbon nanoparticles, averaging 30 to 35 nanometers in diameter, could enter the olfactory nerve in rodents and climb all the way up to the brain. “There is a possibility that because of their small size, nanoparticles can reach sites in the body that large particles cannot, cross barriers, and react,” says Oberdörster.